The invention relates to image forming systems, and more particularly relates to printheads having multiple dielectric layers.
Different printhead technologies in use today in image forming systems create and reproduce images in different ways. Some of these technologies include a process of charging a surface of an image-receiving member. The term image-receiving member includes any suitable structure capable of obtaining and retaining the charged latent image. The image-receiving member can be a drum, a flat or curved dielectric surface, or a flexible dielectric belt, which moves along a predetermined path. The image-receiving member can also comprise liquid crystal, phosphor screen, or similar display panel in which the latent charge image results in a visible image. The image-receiving member typically includes on an exterior surface a material such as a dielectric layer that lends itself to receiving the latent charge image. A number of organic and inorganic materials are suitable for the dielectric layer of the image-receiving member. The suitable materials include glass enamel, anodized and flame or plasma sprayed high-density aluminum oxide, and plastic, including polyamides, nylons, and other tough thermoplastic or thermoset resins, among other materials.
The image-receiving member moves past an image forming device, such as a printhead, which produces streams of accelerated electrons as primary charge carriers. The electrons reach the drum, landing in the form of a latent charge image. The latent charge image then receives a developer material, to develop the image, and the image is then transferred and fused to a medium, such as a sheet of paper, to form a printed document.
The printhead most often includes a film having a multi-electrode structure that defines an array of charge-generating sites. Each of the charge-generating sites, when the electrodes are actuated, generates and directs toward the drum a stream of charge carriers, e.g., electrons, to form a pointwise accumulation of charge on the drum that constitutes the latent image.
A representative printhead generally includes a first collection of drive electrodes, e.g., RF-line electrodes, oriented in a first direction across the direction of printing. A second collection of control electrodes, e.g., finger electrodes, oriented transversely to the drive electrodes, forms spatially separated cross points or intersections with the drive electrodes to form the charge-generating sites, at which charges originate. The electrodes themselves are actually separated and electrically insulated from each other by at least one dielectric layer or composition.
The printhead can also include a second insulating layer and a third electrode structure, often identified as a screen electrode. The second insulating layer couples to the finger electrodes and the screen electrodes. The screen electrodes have a plurality of passages in alignment with the charge-generating sites, to allow the streams of charge carriers to pass through. The screen electrode can be a single conductive sheet having an aperture aligned over each charge-generating site. The polarity of the charge carriers passing through the passages, or apertures, depends on the voltage difference applied to the finger and screen electrodes. The polarity of particles accumulated on the drum to create latent image is determined by the voltage difference between the screen electrode and the drum surface. The charged particles of appropriate polarity are inhibited from passing through the aperture, depending upon the sign of the charge, so that the printhead emits either positive or negative charge carriers, depending on its electrode operating potentials.
A disadvantage of conventional thin film printheads is a significant rate of dielectric erosion caused by reactive ion bombardment of the dielectric surface. The erosion rate of the dielectric is proportional to, among other factors, the sputtering yield, impinging ion fluency, and angle of ion incidence. A known solution for minimizing sputtering yield is to utilize a high density, hard and relatively defect-free, dielectric material. To decrease the impinging ion fluency, a reduction of the dielectric layer capacitance is required.
The particular dielectric material utilized has proven a significant factor in erosion resistance. The erosion rate of aluminum oxide, for example, is substantially less than that of silicon oxide. However, alumina permittivity is more than double the value of silicon dioxide. Therefore, a single aluminum oxide layer used for thin film printhead structures results in an increased capacitance. The high capacitance of the dielectric layer, in turn, leads to a rise of the ion bombardment density and to an increase in the dielectric erosion rate. Furthermore, a high printhead capacitance reduces the printing speed and/or the print resolution.
There exists in the art a need for a thin film printhead having a dielectric layer composition with a relatively small capacitance while also being substantially plasma erosion resistant. The present invention is directed toward further solutions in this art.
A printhead, in accordance with one example embodiment of the present invention, has at least a first electrode layer (e.g., RF-line electrodes) and at least a second electrode layer (e.g., finger electrodes). A dielectric composition constructed of at least two dielectric layers of different dielectric materials insulates the first and second electrode layers with respect to each other. Those of ordinary skill in the art will readily recognize that additional layers can combine to form the printhead. For sake of simplicity, we discuss in detail herein the foregoing three printhead components.
The printhead, according to a further aspect of the present invention, includes an RF-line electrode layer forming the first electrode layer, and a finger electrode layer forming the second electrode layer. The RF-line electrode layer and the finger electrode layer form intersections, defining charge-generating sites for emitting charge carriers. The dielectric composition electrically insulates the RF-line electrode layer from the finger electrode layer by the dielectric composition constructed of at least two dielectric layers of different materials.
The dielectric composition, in accordance with another aspect of the present invention, can have a plurality of dielectric layers utilizing different dielectric materials. These dielectric materials can be, for example, any one of silicon dioxide, aluminum oxide, silicon nitride, magnesium oxide, and boron nitride, among other materials not specified herein. Each of the layers within an individual dielectric composition is comprised of multiple different layers of dielectric material separating the finger and RF-line electrodes.
There are at least two dielectric materials, according to another aspect of the present invention, forming multiple layers in the dielectric composition. The dielectric composition can be formed of any number of materials and layers while still fitting within structural limitations for a printhead within an image forming system. There can be an equal number of dielectric materials forming such layers, or alternatively a lesser number of dielectric materials. The dielectric materials can alternate in arrangement to form the plurality of dielectric layers when there are more dielectric layers than dielectric materials utilized.
At least one dielectric layer, according to still another aspect of the present invention, contains impurities. These impurities can be chosen, for example, from a group consisting of carbon, boron, tungsten, and thallium, among other impurities not specified herein.